SUMMARY OF WORK: Mononuclear cells when stimulated with concanavalin A develop different biolgical activites, including suppressor activity. The objective of this work is to find out whether the inhibitor activity is mediated through soluble factors, and to characterize these factors' biological and physiochemical properties. We have found that the factors responsible for suppression are non dialyzable, heat stable, resistant to pH2 treatment and inhibits proliferation of a variety of cells including human lymphocytes, retinoblastoma cells and stromal keratocytes.